This invention relates to the field of Uninterruptible Power Supplies (UPS) powered primarily from electric utility alternating current (AC) power and providing controlled AC power at their output. UPS usually contain internal electrical storage batteries storing direct current (DC) power, which are used during interruptions of utility power flow. This invention describes a novel way of connecting a battery in UPS circuits.
Five patents relate to the present invention. Each is described separately below.
U.S. Pat. No. 5,017,800 to Divan describes a UPS which has an input converter feeding an electrical storage DC capacitor, an inverter powered from this capacitor, and a battery connected in parallel with the DC storage capacitor. This reference describes classical UPS topology. Some of the disadvantages of this circuit are as follows. The battery is selected on an economic basis depending upon the amount of desired reserve time at a given power level. The DC voltage of this battery is usually much smaller than the DC voltage on the capacitors. The battery can not be connected in parallel with the capacitors because a high voltage on the capacitors will either damage or destroy the battery. Therefore, Divan uses a DC-DC boost converter to boost the battery voltage to the level of voltage on the capacitors. See FIG. 6 of this patent. This circuit increases the cost of the UPS because it adds this boost converter which is used only on an emergency basis when the utility power fails.
U.S. Pat. No. 4,935,861 to Johnson et al. describes another UPS with fewer advantages than Divan. Johnson also uses classical UPS topology with the battery connected to DC capacitors located between a rectifier and an inverter. In this invention, two DC capacitors are used, each in a half wave rectifier. Then again, a boost DC-DC converter is used to boost the battery voltage to the voltage level on the capacitors, e.g., from 12 V to 170 V. But because there are two capacitors with different voltage polarities and common connections to the battery, a boost converter is used to boost voltage to one capacitor in the same polarity as the battery voltage, and use of a transformer with an inverted polarity to the other capacitor. This converter is even more disadvantageous because it is more complex than Divan's, i.e., has more parts including a half power transformer, and therefore is even more expensive.
U.S. Pat. No. 4,827,150 to Reynal describes another UPS with classical topology. Again, a battery is connected to capacitors via a boost converter.
U.S. Pat. No. 4,779,007 to Schlanger et al. describes a different UPS topology. In this UPS, power to an inverter is delivered via a switch between DC power on the output of a rectifier and DC power on the output of an "up converter", i.e., a boost DC-DC converter. This topology does not eliminate the disadvantages of having a boost converter to boost the battery voltage to the level produced by an AC rectifier. Therefore, this invention retains the cost disadvantage of having full power converter which is rarely used.
U.S. Pat. No. 4,277,692 to Small describes an alternate UPS topology. In Small, power flows from the utility AC source to the load via a switch. A bidirectional power converter is located in parallel with the load. When utility power is present, this converter converts AC into DC and charges the battery. When utility power fails, the switch connecting this power to the load opens and the converter inverts DC power from the battery to an AC power to the load. However, this topology has major performance disadvantages over the topologies described above. Other topologies, called "On-Line UPS", use an inverter to provide power to the load at all times. Because the inverter operation is totally internally controlled, it provides controlled regulated power to the load. In the "Stand-By" topology described by the Small patent, when the utility power fails, the switch requires a finite time to turn off and reverse power flow in the bidirectional converter. During this interval, there is no power flow to a load. This time is typically larger than 4-6 millisecond, which is large in comparison with a half cycle of utility power (8.3 to 10 milliseconds). Numerous loads do not tolerate such long interruptions of power flow. Additionally, all waveform distortions shorter in duration than this switchover time propagate to the load, distortions such as spikes, oscillations, and EMI noise.